Critical capillary number, mobilization

In the case of discontinuous oil, L may be equal to Db, which is the diameter of a single oil blob. The capillary number required to mobilize the single oil blob is calculated using the preceding equation with L = Db. In the case of continuous oil whose size could be several times of Db, and L would be several times of Db, then the capillary number required to mobihze the continuous oil would be several times lower than that required to mobilize a single oil blob. In other words, the critical capillary number required to mobilize discontinuous oil is higher than that to mobilize continuous oil. This is another justification that chemical flood should be conducted early in the secondary recovery mode instead of in the tertiary recovery mode. 

To further reduce waterflood residual oil saturation, the capillary number must be higher than the preceding calculated value. In general, the capillary number must be higher than a critical capillary number, (Nc)c, for a residual phase to start to mobilize. Practically, this (Nc)c is much higher than the capillary number at normal waterflooding conditions. Another parameter is maximum desaturation capillary number, (Nc)max, above which the residual saturation would not be further reduced in practical conditions even if the capillary number is increased. Lake (1989) used the term total desaturation capillary number for (Nc)max. In practical conditions, total desaturation (i.e., zero residual saturation) may not occur due to some films or blobs trapped in pores. 

Morrow and coworkers (Morrow and Songkran, 1981 Morrow et al., 1988) used the terms capillary number for mobilization and capillary number for prevention of entrapment for (Nc)c and (Nc)max, respectively. In UTCHEM, lower and higher critical capillary numbers are used for (Nc)c and (Nc)max, respectively. Table 7.8 summarizes some of the published experimental data for these critical capillary numbers. In principle, the critical capillary numbers should be system specific. Experiments should always be conducted to determine the capillary desaturation curves (CDC) for the particular application whenever possible. The summarized data could be useful only when no experimental data are available. From Table 7.8, the following observations can be made regarding capillary number ... 

The critical capillary number required to mobilize discontinuous oil is higher than that to mobilize continuous oil. 

Now we have discussed the two important capillary numbers critical and maximum. The general relationship between residual saturation of a nonaque-ous or aqueous phase and a local capillary number is called capillary desaturation curve (CDC). The residual saturations start to decrease at the critical capillary number as the capillary number increases, and cannot be decreased further at the maximum capillary number. As discussed earlier, the range of capillary numbers for residual phases to be mobilized is, for example, 10 to... 

Dispersion Formation, Subdivision, and Coalescence. The ability to create and control dispersions at distances far from the injection well will be critical to the field-use of dispersion-based mobility control. The early studies of Bernard and Holm, followed by more recent work by Hirasaki, Falls, and co-workers, and others showed that the flow properties of surfactant-induced dispersions depend on the presence and composition of oil, volume ratio of the dispersed and continuous phases, capillary pressure, and capillary number (35,37,39-41,52-54,68). However, it is the size of the droplets or bubbles that dominates dispersion flow (39,68). Moreover, early debates on the ratio of droplet (or bubble) size to pore size have been resolved by ample evidence showing that, under commonly employed conditions, droplets are larger than pores (39). Only for very large capillary numbers (i.e., for interfacial tensions of ca. 

The filled circles in Eigure 7.29 represent the experimental data of Ap/(Lo) at different permeabilities reported by Taber et al. (1973). These data, the critical valnes at which the residual oil saturation started to decrease, show a declining trend with permeability. However, when these data are converted to kjAp/ (Lo), which is the capillary number defined in Eq. 7.85, and plotted in the same figure (empty circles), we can see that all the data are near a horizontal fine at 1500 (md psi/ft)/(mN/m), which is equivalent to the dimensionless capillary number 3.4 x 10, as reported in Table 7.7. This result further suggests that we should use Eq. 7.85 because the capillary number required to mobilize... 

---